I get the impression that one could just write the total amount of deductions in one line and be done with it. In the event of an audit the taxpayer should be able to produce the fuller details.

My lawyers and accountants have said some things that seem to run counter to how I colloquially interpreted the word "itemize", and I now realize that is merely a regulatory term that simply means "distinct from standard deduction" but not necessarily "the verb form of listing all items".

Search engines dont seem to help here, because they are just hundreds of personal finance blogs also giving very rudimentary information on itemizing VS standard deductions.

1 Answer
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When you do your taxes, you have two choices for your deductions. You can take the standard deduction, or you can choose to itemize your deductions. If you itemize your deductions, you use Form 1040 Schedule A.

By looking at Schedule A, you can see the list of deductions that are itemized:

Medical expenses

Taxes you paid (state taxes, property taxes, other taxes)

Interest you paid (home mortgage interest, investment interest)

Gifts to charity

Casualty and theft losses

Job expenses and certain miscellaneous deductions

Other miscellaneous deductions

On Schedule A itself, you only list a total for each of these broad categories. In some cases, this is sufficient detail. However, for certain deductions, finer detail may be required, and you may have to submit additional forms showing this detail. For example, on the medical expense line, you generally only list a total of medical expenses; details are only supplied to the IRS upon request. For noncash gifts to charity, you need to supply more details on Form 8283 if your gifts are worth more than $500.

As noted by @Accumulation in the comments, the above deductions that are a part of your itemized deductions are called "below the line" deductions (because they are subtracted after the adjusted gross income line) and are only able to be deducted if you choose to decline the standard deduction. There are other deductions that are available whether or not you itemize. These "above the line" deductions are found on Form 1040 Lines 23-35. If you look at these lines on the form, you'll see the different types of deductions that are called out here. Some of these deductions require additional details on other forms; for example, the HSA deduction requires details on Form 8889.

If you have a business, your business expenses are not part of your itemized deductions at all, and do not appear on Schedule A anywhere. Instead, your business expenses get subtracted from your business's revenue, and the resulting profit (or loss) is what is reported on your Form 1040.

Different types of businesses report these expenses differently. If you have a sole proprietorship, the details of your business's expenses are reported on Schedule C. On this schedule, Part II is devoted to deductible business expenses. Take a look at Schedule C, and you'll see that Lines 8-27 are different categories of expenses that get called out on this schedule.

Also, there is a distinction between "above the line" deductions and "below the line" deductions. The latter are taken instead of the standard deductions, while the former are taken in addition to the itemized deductions. Health Savings Accounts, for instance, are above the line deductions. Many above the line deductions will be deducted from your take-home pay, in which case your W-2 should be sufficient documentation.
– AcccumulationNov 10 '17 at 6:02

How about for deductions related to an entreprenurial endeavor? The “networking expenses” and office supplies, all aggregated to a single line?
– CQMNov 10 '17 at 6:44